If a binocular or biocular HMD (head-mounted display or helmet-mounted display) is well constructed and aligned, only one scene is perceived and viewing is comfortable. However, all binocular instruments have slight differences between the two images. The differences in image geometry may have many different causes. One possibility is that the HMD does not fit well and for this reason the displays are not directly in front of the left and right eyes. This means that the eyes are forced to align themselves in an unnatural way, which may cause eyestrain or other severe symptoms. For example, if the interocular distance (IOD) is larger than the interpupillary distance (IPD), the eyes have to turn toward the temples, as shown in FIG. 1a. This would cause eyestrain. Similarly, if one display is positioned higher than the other, the eyes have to look at different vertical levels. This eye movement also strains the muscular systems that support the movements. Misalignment in optical components relative to the eyes can cause image distortion. For example, prismatic effects may occur because of a certain misalignment, as shown in FIG. 2. Optical effects such as towel distortion, rotations, translations and so forth may cause further eyestrains in difficulties in perceiving the image.
Small misalignment or distortions is unnoticeable and may be tolerable, but when misalignment or distortion increases, extended use can cause severe headache. Very large differences between left and right images can cause eyestrain, nausea and headache. The scene will occasionally split into two images or mis-registered images, causing double vision (diplopia). It is also possible that one image is suppressed.
To detect the different types of misalignment, several solutions have been suggested. As shown in FIGS. 3b, 3c and 3d, lines and boxes in the right and left images are used to align the images. Aligning lines or boxes have some disadvantages. These methods can only detect global misalignment. They are unable to detect local differences or complex distortions. In principle, local differences and more complex distortions can be detected by using smaller boxes in smaller versions of alignment procedure. In practice, localized alignments based on local differences are not useful because the visual system is not sensitive enough for such procedures. Furthermore, the alignment-based procedure can make measurements in one direction at a time. For example, it cannot make measurements in both horizontal and vertical directions at the same time. This restriction has two consequences: 1) an assumption about the direction of the misalignment must be made prior to the measurements, and 2) a separate testing sequence is required for each direction if multiple misalignment measurements are desired. This method is not flexible and also time-consuming.
It is thus advantageous and desirable to provide a method and system for detecting local misalignment or complex distortion in a faster and more sensitive, and processes for correcting the misalignments based on the detection.